1. Field of the Invention
The present invention relates to a method for preparing dimethylether from methanol using a membrane reactor, more particularly to a method for preparing dimethylether from methanol using a membrane reactor which is capable of performing a reaction and a separation simultaneously. Since water vapor generated by catalytic reaction can be selectively removed from the reaction zone, decrease in catalytic activity can be prevented and thus a good reaction yield can be obtained even in a mild temperature condition. Further, no additional steps of dimethylether separation and purification are required.
2. Description of the Related Art
In manufacturing chemical products, the steps of a reaction and a separation take significant part of the process, which also requires enormous amount of energy-consumption. If a reaction and a separation steps can be carried out simultaneously using a membrane reactor, the resulting productivity can be much improved. Further, a membrane reactor can improve reaction yield by selectively removing specific substances generated during a reversible reaction with a membrane, thereby increasing conversion beyond thermodynamic equilibrium and reaction efficiency by inhibiting generation of by-products [H. P. Hsieh, xe2x80x9cInorganic Membranes for Separation and Reactionxe2x80x9d, Elsevier, N L, 1996].
The membrane technique is finding its use in various fields, including environmental and energy industries, chemical industries, biological industries and semiconductor industries, as a new technology enabling separation and purification with low energy consumption. Recently, development of a membrane reactor, which can greatly improve reaction yield by combination of reaction and separation steps, has been drawing much attention.
Dimethylether (DME), a prospective alternative fuel for a diesel engine, is being spotlighted as a clean transportation energy. At present, DME, the future clean fuel is produced from methanol in traditional catalyst reactors. DME emits much less nitrogen oxides than gasoline and diesel oils while having thermal efficiency still comparable to theirs. Also, it hardly causes smog and makes little noise. Accordingly, it is expected to be used in large scale as an alternative fuel for diesel oils and LPG, which account for more than 30% of the current transportation fuels in Korea. In this regard, establishment of an effective and stable mass-production system is drawing much attention. Production of dimethylether by dehydration of methanol can be represented by the following reaction formula.
2CH3OH⇄CH3OCH3+H2O
If a traditional catalyst reactor is used, the above reaction cannot proceed further once the reaction reaches thermodynamic equilibrium. Therefore, conversion of methanol and yield of DME are greatly limited. And, a complex distillation step, which consumes a lot of energy, is necessary to remove unreacted methanol and water vapor from the product to obtain high-purity DME. That is, the traditional method had to run the reactor at a high temperature of over 300xc2x0 C. and a high pressure of over 10 atm to improve conversion and reaction yield. Water vapor generated during catalytic dehydration of methanol is known to decrease catalytic activity of alumina-silica catalyst. Therefore, improvement of catalytic reaction efficiency and simplification of process are required for a stable and effective DME mass-production system.
The inventors of the present invention have identified that when a membrane reactor capable of combining reaction and separation steps is used in preparation of DME from methanol, water vapor that may reduce catalytic activity can be selectively removed as soon as it is generated. Consequently, the life of a catalyst can be prolonged and reaction yield can be improved at a milder temperature and pressure condition compared to the conventional ones. Also, additional separation and purification steps are no longer required.
Accordingly, an object of the present invention is to provide a new membrane reactor capable of separating unreacted materials and impurities from a product and selectively removing them as soon as they are generated, and a method for preparing DME using the same.